Flow measurement for a gas turbine engine

ABSTRACT

A flow machine having a flow passage and an air flow measurement system comprising a plurality of acoustic sensors. The acoustic sensors comprise at least one acoustic transmitter configured to transmit an acoustic waveform through the airflow passing through the flow passage to an acoustic receiver. At least one of the acoustic sensors is rotatable relative to one or more other acoustic sensor.

BACKGROUND Field of the Disclosure

The present disclosure relates to an acoustic measurement system,particularly to an acoustic measurement system for a turbomachine, suchas an axial flow engine.

Description of the Related Art

In turbomachines, for example gas turbine engines, measurement of theproperties of the air flowing into or through the engine can be used todetermine the performance of the engine. For example, the average flowvelocity or the mass flow of the airflow can be used to calculate theperformance and/or efficiency of the engine.

Measurements of total and static pressure may be performed usingconventional devices, such as Pitot tubes. Pitot tubes comprise anaperture configured to face into the oncoming airflow and mustprotrude/extend into the airflow, for example, to avoid a boundaryairflow layer in an inner surface of the casing/ducting surrounding theairflow. Other devices, such as static ports must be mounted within thecasing.

The extent which the Pitot tubes extend into the airflow is restrictedin order to reduce disruption to the airflow, thus the Pitot tubes canonly sample the airflow properties in a limited region of the airflow.

Sampled measurements made by the Pitot tubes in the limited region ofthe airflow are assumed to have a correlation with the overall (bulk)airflow flowing into/through the engine. However, the correlation mustbe modeled or estimated to calculate the bulk airflow from the sampledmeasurements. Furthermore, the samples in the limited region may not berepresentative of the bulk airflow, for example, it may be influenced bythe boundary layer of the airflow along the casing. Therefore, thecorrelation model may not be robust to a changing operating conditionand so the calculation of the bulk airflow may not be accurate.

Another problem faced when attempting to measure flow rates on axialflow machines such as gas turbine engines is that compressibilityeffects of the flow become relevant for high volumetric flow rates. Thiscomplicates models that might need to be used to correlate sensorreadings to overall/bulk flow rate values.

Conventional models rely on velocity profiles attained during analyticalor experimental calibration and do not account for unassessedconditions. Also, average pressures and temperatures are used todetermine mass flow rate from volumetric flow rate. However, theconditions for a flow regime within the interior of an engine are highlydynamic.

The present disclosure aims to overcome or ameliorate one or more of theabove problems.

SUMMARY OF THE DISCLOSURE

According to a first aspect of the disclosure there is provided a flowmachine having a flow passage and an air flow measurement system, theair flow measurement system comprising: a plurality of acoustic sensors;the acoustic sensors comprising at least one acoustic transmitterconfigured to transmit an acoustic waveform through the airflow passingthrough the flow passage to an acoustic receiver; and where at least oneof the acoustic sensors is rotatable relative to one or more otheracoustic sensor.

The at least one acoustic sensor may be rotatable about the one or moreother acoustic sensor in use, e.g. so as to sweep through an arc orrevolution about the one or more further acoustic sensor. The at leastone acoustic sensor may sweep through an arc or revolution defining avolume/area of the airflow over which air flow measurement is performed.

The flow machine may comprise a compressor rotatable about an axis, theleast one rotatable acoustic sensor being rotatable about said axis.

At least one acoustic sensor may be provided at a first radial distancefrom said axis and at least one acoustic sensor is provided at a second,greater radial distance from said axis.

At least one acoustic sensor may be mounted on a rotatable portion ofthe flow machine within the flow passage and at least one acousticsensor may be mounted at a position surrounding said rotatable portion.

At least one acoustic sensor may be mounted on a casing of the flowpassage, and at least one acoustic sensor may be mounted to a rotatablehub or shaft provided within the casing.

The rotatable hub may comprise a compressor fan hub.

A plurality of acoustic sensors may be mounted on the rotatable portionand/or a plurality of acoustic sensors may be mounted on the casing.

The plurality of acoustic sensors may be circumferentially spaced on therotatable portion and/or casing respectively.

The respective lines of sight between each of the acoustic sensorsprovided on the rotatable hub and the acoustic sensors provided on thecasing may substantially span the entire flow area between the hub andthe casing at a given time frame.

At least one acoustic sensor may be recessed or mounted flush on therotatable portion and/or casing.

The acoustic sensors may be provided at an intake of the flow passageand/or upstream of a rotor of the flow machine.

The rotatable acoustic sensor may be configured to sweep an area of theintake.

The rotatable acoustic sensor may be configured to sweep an entire flowarea between the hub and the casing during a revolution thereof.

The rotatable acoustic sensor may be rotatable about an axissubstantially parallel to any or any combination of a central axis ofthe flow passage; the direction of air flow through the flow passageand/or a rotational axis of the flow machine.

The acoustic sensors may all be arranged in a single plane.

The at least one acoustic sensor may be wirelessly electrically coupledto allow electrical power or electrical communication to be providedthereto.

The at least one acoustic sensor may be electrically coupled viarotating electrical interface to allow electrical power or electricalcommunication to be provided thereto.

The flow machine may be a turbomachine or a gas turbine engine.

According to a second aspect of the disclosure there is provided asystem configured to determine air flow through a flow machine having aflow passage, the system comprising: a plurality of acoustic sensors,the acoustic sensors comprising at least one acoustic transmitterconfigured to transmit an acoustic waveform through the airflow passingthrough the flow passage to an acoustic receiver, where at least one ofthe acoustic sensors is rotatable relative to one or more other acousticsensor; and a processing system configured to receive signals from theacoustic sensors and determine a flow rate of the flow through the flowpassage.

According to a third aspect of the disclosure there is provided a methodof determining air flow properties of an airflow through a flow passageof a flow machine, the method comprising the steps of: providing signalcommunication with a plurality of acoustic sensors mounted relative tothe flow passage, where at least one of the acoustic sensors isrotatable relative to one or more other acoustic sensor; determining atime of flight of an acoustic waveform between said plurality ofacoustic sensors; and using the time of flight between the plurality ofsensors to determine an average flow velocity of the airflow through theflow passage.

As noted elsewhere herein, the present disclosure may relate to a gasturbine engine. Such a gas turbine engine may comprise an engine corecomprising a turbine, a combustor, a compressor, and a core shaftconnecting the turbine to the compressor. Such a gas turbine engine maycomprise a fan (having fan blades) located upstream of the engine core.

Arrangements of the present disclosure may be particularly, although notexclusively, beneficial for fans that are driven via a gearbox.Accordingly, the gas turbine engine may comprise a gearbox that receivesan input from the core shaft and outputs drive to the fan so as to drivethe fan at a lower rotational speed than the core shaft. The input tothe gearbox may be directly from the core shaft, or indirectly from thecore shaft, for example via a spur shaft and/or gear. The core shaft mayrigidly connect the turbine and the compressor, such that the turbineand compressor rotate at the same speed (with the fan rotating at alower speed).

The gas turbine engine as described and/or claimed herein may have anysuitable general architecture. For example, the gas turbine engine mayhave any desired number of shafts that connect turbines and compressors,for example one, two or three shafts. Purely by way of example, theturbine connected to the core shaft may be a first turbine, thecompressor connected to the core shaft may be a first compressor, andthe core shaft may be a first core shaft. The engine core may furthercomprise a second turbine, a second compressor, and a second core shaftconnecting the second turbine to the second compressor. The secondturbine, second compressor, and second core shaft may be arranged torotate at a higher rotational speed than the first core shaft.

In such an arrangement, the second compressor may be positioned axiallydownstream of the first compressor. The second compressor may bearranged to receive (for example directly receive, for example via agenerally annular duct) flow from the first compressor.

The gearbox may be arranged to be driven by the core shaft that isconfigured to rotate (for example in use) at the lowest rotational speed(for example the first core shaft in the example above). For example,the gearbox may be arranged to be driven only by the core shaft that isconfigured to rotate (for example in use) at the lowest rotational speed(for example only be the first core shaft, and not the second coreshaft, in the example above). Alternatively, the gearbox may be arrangedto be driven by any one or more shafts, for example the first and/orsecond shafts in the example above.

The gearbox may be a reduction gearbox (in that the output to the fan isa lower rotational rate than the input from the core shaft). Any type ofgearbox may be used. For example, the gearbox may be a “planetary” or“star” gearbox, as described in more detail elsewhere herein. Thegearbox may have any desired reduction ratio (defined as the rotationalspeed of the input shaft divided by the rotational speed of the outputshaft), for example greater than 2.5, for example in the range of from 3to 4.2, or 3.2 to 3.8, for example on the order of or at least 3, 3.1,3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1 or 4.2. The gear ratiomay be, for example, between any two of the values in the previoussentence. Purely by way of example, the gearbox may be a “star” gearboxhaving a ratio in the range of from 3.1 or 3.2 to 3.8. In somearrangements, the gear ratio may be outside these ranges.

In any gas turbine engine as described and/or claimed herein, acombustor may be provided axially downstream of the fan andcompressor(s). For example, the combustor may be directly downstream of(for example at the exit of) the second compressor, where a secondcompressor is provided. By way of further example, the flow at the exitto the combustor may be provided to the inlet of the second turbine,where a second turbine is provided. The combustor may be providedupstream of the turbine(s).

The or each compressor (for example the first compressor and secondcompressor as described above) may comprise any number of stages, forexample multiple stages. Each stage may comprise a row of rotor bladesand a row of stator vanes, which may be variable stator vanes (in thattheir angle of incidence may be variable). The row of rotor blades andthe row of stator vanes may be axially offset from each other.

The or each turbine (for example the first turbine and second turbine asdescribed above) may comprise any number of stages, for example multiplestages. Each stage may comprise a row of rotor blades and a row ofstator vanes. The row of rotor blades and the row of stator vanes may beaxially offset from each other.

Each fan blade may be defined as having a radial span extending from aroot (or hub) at a radially inner gas-washed location, or 0% spanposition, to a tip at a 100% span position. The ratio of the radius ofthe fan blade at the hub to the radius of the fan blade at the tip maybe less than (or on the order of) any of: 0.4, 0.39, 0.38 0.37, 0.36,0.35, 0.34, 0.33, 0.32, 0.31, 0.3, 0.29, 0.28, 0.27, 0.26, or 0.25. Theratio of the radius of the fan blade at the hub to the radius of the fanblade at the tip may be in an inclusive range bounded by any two of thevalues in the previous sentence (i.e. the values may form upper or lowerbounds), for example in the range of from 0.28 to 0.32. These ratios maycommonly be referred to as the hub-to-tip ratio. The radius at the huband the radius at the tip may both be measured at the leading edge (oraxially forwardmost) part of the blade. The hub-to-tip ratio refers, ofcourse, to the gas-washed portion of the fan blade, i.e. the portionradially outside any platform.

The radius of the fan may be measured between the engine centreline andthe tip of a fan blade at its leading edge. The fan diameter (which maysimply be twice the radius of the fan) may be greater than (or on theorder of) any of: 220 cm, 230 cm, 240 cm, 250 cm (around 100 inches),260 cm, 270 cm (around 105 inches), 280 cm (around 110 inches), 290 cm(around 115 inches), 300 cm (around 120 inches), 310 cm, 320 cm (around125 inches), 330 cm (around 130 inches), 340 cm (around 135 inches),350cm, 360cm (around 140 inches), 370 cm (around 145 inches), 380(around 150 inches) cm, 390 cm (around 155 inches), 400 cm, 410 cm(around 160 inches) or 420 cm (around 165 inches). The fan diameter maybe in an inclusive range bounded by any two of the values in theprevious sentence (i.e. the values may form upper or lower bounds), forexample in the range of from 240 cm to 280 cm or 330 cm to 380 cm.

The rotational speed of the fan may vary in use. Generally, therotational speed is lower for fans with a higher diameter. Purely by wayof non-limitative example, the rotational speed of the fan at cruiseconditions may be less than 2500 rpm, for example less than 2300 rpm.Purely by way of further non-limitative example, the rotational speed ofthe fan at cruise conditions for an engine having a fan diameter in therange of from 220 cm to 300 cm (for example 240 cm to 280 cm or 250 cmto 270cm) may be in the range of from 1700 rpm to 2500 rpm, for examplein the range of from 1800 rpm to 2300 rpm, for example in the range offrom 1900 rpm to 2100 rpm. Purely by way of further non-limitativeexample, the rotational speed of the fan at cruise conditions for anengine having a fan diameter in the range of from 330 cm to 380 cm maybe in the range of from 1200 rpm to 2000 rpm, for example in the rangeof from 1300 rpm to 1800 rpm, for example in the range of from 1400 rpmto 1800 rpm.

In use of the gas turbine engine, the fan (with associated fan blades)rotates about a rotational axis. This rotation results in the tip of thefan blade moving with a velocity U_(tip). The work done by the fanblades 13 on the flow results in an enthalpy rise dH of the flow. A fantip loading may be defined as dH/U_(tip) ², where dH is the enthalpyrise (for example the 1-D average enthalpy rise) across the fan andU_(tip) is the (translational) velocity of the fan tip, for example atthe leading edge of the tip (which may be defined as fan tip radius atleading edge multiplied by angular speed). The fan tip loading at cruiseconditions may be greater than (or on the order of) any of: 0.28, 0.29,0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39 or 0.4 (allunits in this paragraph being Jkg⁻¹K⁻¹/(ms⁻¹)²). The fan tip loading maybe in an inclusive range bounded by any two of the values in theprevious sentence (i.e. the values may form upper or lower bounds), forexample in the range of from 0.28 to 0.31, or 0.29 to 0.3.

Gas turbine engines in accordance with the present disclosure may haveany desired bypass ratio, where the bypass ratio is defined as the ratioof the mass flow rate of the flow through the bypass duct to the massflow rate of the flow through the core at cruise conditions. In somearrangements the bypass ratio may be greater than (or on the order of)any of the following: 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5,15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5 or 20. The bypass ratiomay be in an inclusive range bounded by any two of the values in theprevious sentence (i.e. the values may form upper or lower bounds), forexample in the range of form 12 to 16, 13 to 15, or 13 to 14. The bypassduct may be substantially annular. The bypass duct may be radiallyoutside the core engine. The radially outer surface of the bypass ductmay be defined by a nacelle and/or a fan case.

The overall pressure ratio of a gas turbine engine as described and/orclaimed herein may be defined as the ratio of the stagnation pressureupstream of the fan to the stagnation pressure at the exit of thehighest pressure compressor (before entry into the combustor). By way ofnon-limitative example, the overall pressure ratio of a gas turbineengine as described and/or claimed herein at cruise may be greater than(or on the order of) any of the following: 35, 40, 45, 50, 55, 60, 65,70, 75. The overall pressure ratio may be in an inclusive range boundedby any two of the values in the previous sentence (i.e. the values mayform upper or lower bounds), for example in the range of from 50 to 70.

Specific thrust of an engine may be defined as the net thrust of theengine divided by the total mass flow through the engine. At cruiseconditions, the specific thrust of an engine described and/or claimedherein may be less than (or on the order of) any of the following: 110Nkg⁻¹s, 105 Nkg⁻¹s, 100 Nkg⁻¹s, 95 Nkg⁻¹s, 90 Nkg⁻¹s, 85 Nkg⁻¹s or 80Nkg⁻¹s. The specific thrust may be in an inclusive range bounded by anytwo of the values in the previous sentence (i.e. the values may formupper or lower bounds), for example in the range of from 80 Nkg⁻¹s to100 Nkg⁻¹s, or 85 Nkg⁻¹s to 95 Nkg⁻¹s. Such engines may be particularlyefficient in comparison with conventional gas turbine engines.

A gas turbine engine as described and/or claimed herein may have anydesired maximum thrust. Purely by way of non-limitative example, a gasturbine as described and/or claimed herein may be capable of producing amaximum thrust of at least (or on the order of) any of the following:160 kN, 170 kN, 180 kN, 190 kN, 200 kN, 250 kN, 300 kN, 350 kN, 400 kN,450 kN, 500 kN, or 550 kN. The maximum thrust may be in an inclusiverange bounded by any two of the values in the previous sentence (i.e.the values may form upper or lower bounds). Purely by way of example, agas turbine as described and/or claimed herein may be capable ofproducing a maximum thrust in the range of from 330 kN to 420 kN, forexample 350 kN to 400 kN. The thrust referred to above may be themaximum net thrust at standard atmospheric conditions at sea level plus15 degrees C. (ambient pressure 101.3 kPa, temperature 30 degrees C.),with the engine static.

In use, the temperature of the flow at the entry to the high pressureturbine may be particularly high. This temperature, which may bereferred to as TET, may be measured at the exit to the combustor, forexample immediately upstream of the first turbine vane, which itself maybe referred to as a nozzle guide vane. At cruise, the TET may be atleast (or on the order of) any of the following: 1400K, 1450K, 1500K,1550K, 1600K or 1650K. The TET at cruise may be in an inclusive rangebounded by any two of the values in the previous sentence (i.e. thevalues may form upper or lower bounds). The maximum TET in use of theengine may be, for example, at least (or on the order of) any of thefollowing: 1700K, 1750K, 1800K, 1850K, 1900K, 1950K or 2000K. Themaximum TET may be in an inclusive range bounded by any two of thevalues in the previous sentence (i.e. the values may form upper or lowerbounds), for example in the range of from 1800K to 1950K. The maximumTET may occur, for example, at a high thrust condition, for example at amaximum take-off (MTO) condition.

A fan blade and/or aerofoil portion of a fan blade described and/orclaimed herein may be manufactured from any suitable material orcombination of materials. For example at least a part of the fan bladeand/or aerofoil may be manufactured at least in part from a composite,for example a metal matrix composite and/or an organic matrix composite,such as carbon fibre. By way of further example at least a part of thefan blade and/or aerofoil may be manufactured at least in part from ametal, such as a titanium based metal or an aluminium based material(such as an aluminium-lithium alloy) or a steel based material. The fanblade may comprise at least two regions manufactured using differentmaterials. For example, the fan blade may have a protective leadingedge, which may be manufactured using a material that is better able toresist impact (for example from birds, ice or other material) than therest of the blade. Such a leading edge may, for example, be manufacturedusing titanium or a titanium-based alloy. Thus, purely by way ofexample, the fan blade may have a carbon-fibre or aluminium based body(such as an aluminium lithium alloy) with a titanium leading edge.

A fan as described and/or claimed herein may comprise a central portion,from which the fan blades may extend, for example in a radial direction.The fan blades may be attached to the central portion in any desiredmanner. For example, each fan blade may comprise a fixture which mayengage a corresponding slot in the hub (or disc). Purely by way ofexample, such a fixture may be in the form of a dovetail that may slotinto and/or engage a corresponding slot in the hub/disc in order to fixthe fan blade to the hub/disc. By way of further example, the fan bladesmaybe formed integrally with a central portion. Such an arrangement maybe referred to as a bladed disc or a bladed ring. Any suitable methodmay be used to manufacture such a bladed disc or bladed ring. Forexample, at least a part of the fan blades may be machined from a blockand/or at least part of the fan blades may be attached to the hub/discby welding, such as linear friction welding.

The gas turbine engines described and/or claimed herein may or may notbe provided with a variable area nozzle (VAN). Such a variable areanozzle may allow the exit area of the bypass duct to be varied in use.The general principles of the present disclosure may apply to engineswith or without a VAN.

The fan of a gas turbine as described and/or claimed herein may have anydesired number of fan blades, for example 14, 16, 18, 20, 22, 24 or 26fan blades.

As used herein, cruise conditions may mean cruise conditions of anaircraft to which the gas turbine engine is attached. Such cruiseconditions may be conventionally defined as the conditions atmid-cruise, for example the conditions experienced by the aircraftand/or engine at the midpoint (in terms of time and/or distance) betweentop of climb and start of decent.

Purely by way of example, the forward speed at the cruise condition maybe any point in the range of from Mach 0.7 to 0.9, for example 0.75 to0.85, for example 0.76 to 0.84, for example 0.77 to 0.83, for example0.78 to 0.82, for example 0.79 to 0.81, for example on the order of Mach0.8, on the order of Mach 0.85 or in the range of from 0.8 to 0.85. Anysingle speed within these ranges may be the cruise condition. For someaircraft, the cruise conditions may be outside these ranges, for examplebelow Mach 0.7 or above Mach 0.9.

Purely by way of example, the cruise conditions may correspond tostandard atmospheric conditions at an altitude that is in the range offrom 10000 m to 15000 m, for example in the range of from 10000 m to12000 m, for example in the range of from 10400 m to 11600 m (around38000 ft), for example in the range of from 10500 m to 11500 m, forexample in the range of from 10600 m to 11400 m, for example in therange of from 10700 m (around 35000 ft) to 11300 m, for example in therange of from 10800 m to 11200 m, for example in the range of from 10900m to 11100 m, for example on the order of 11000 m. The cruise conditionsmay correspond to standard atmospheric conditions at any given altitudein these ranges.

Purely by way of example, the cruise conditions may correspond to: aforward Mach number of 0.8; a pressure of 23000 Pa; and a temperature of−55 degrees C. Purely by way of further example, the cruise conditionsmay correspond to: a forward Mach number of 0.85; a pressure of 24000Pa; and a temperature of −54 degrees C. (which may be standardatmospheric conditions at 35000 ft).

As used anywhere herein, “cruise” or “cruise conditions” may mean theaerodynamic design point. Such an aerodynamic design point (or ADP) maycorrespond to the conditions (comprising, for example, one or more ofthe Mach Number, environmental conditions and thrust requirement) forwhich the fan is designed to operate. This may mean, for example, theconditions at which the fan (or gas turbine engine) is designed to haveoptimum efficiency.

In use, a gas turbine engine described and/or claimed herein may operateat the cruise conditions defined elsewhere herein. Such cruiseconditions may be determined by the cruise conditions (for example themid-cruise conditions) of an aircraft to which at least one (for example2 or 4) gas turbine engine may be mounted in order to provide propulsivethrust.

The skilled person will appreciate that except where mutually exclusive,a feature or parameter described in relation to any one of the aboveaspects may be applied to any other aspect. Furthermore, except wheremutually exclusive, any feature or parameter described herein may beapplied to any aspect and/or combined with any other feature orparameter described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with referenceto the Figures, in which:

FIG. 1 is a sectional side view of a gas turbine engine;

FIG. 2 is a close up sectional side view of an upstream portion of a gasturbine engine;

FIG. 3 is a partially cut-away view of a gearbox for a gas turbineengine;

FIG. 4 is a schematic front view of an intake of a gas turbine enginehaving acoustic flow sensors;

FIG. 5 is a schematic sectional view of an intake of a gas turbineengine having acoustic flow sensors;

FIG. 6 shows an example acoustic sensor array and the pathsthere-between;

FIG. 7 shows a further example acoustic sensor array for a differentflow path;

FIG. 8 shows a further example acoustic sensor array;

FIG. 9 shows a further example acoustic sensor array accommodatingrelative rotation;

FIG. 10 is a schematic sectional view of a further example of an intakeof a gas turbine engine having acoustic flow sensors;

FIG. 11 shows a schematic section view through an acoustic sensor array,indicating parameters used in examples of flow measurement during use.

FIG. 12 shows a flowchart of the estimation of one or more engineparameters.

FIG. 13 is a sectional side view of a bypass for a gas turbine engine.

DETAILED DESCRIPTION OF THE DISCLOSURE

Embodiments will now be described by way of example only, with referenceto the Figures.

FIG. 1 illustrates a gas turbine engine 10 having a principal rotationalaxis 9. The engine 10 comprises an air intake 12 which receives anintake airflow 48 and a propulsive fan 23 that generates two airflows: acore airflow A and a bypass airflow B. The intake airflow 48 comprisesthe sum total of the air flowing into the operational upstream end ofthe engine 10, with the sum total of the core airflow A and the bypassairflow B substantially equal to the intake airflow 48.

The gas turbine engine 10 comprises a core 11 that receives the coreairflow A. The engine core 11 comprises, in axial flow series, a lowpressure compressor 14, a high-pressure compressor 15, combustionequipment 16, a high-pressure turbine 17, a low pressure turbine 19 anda core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. Thebypass airflow B flows through the bypass duct 22. The fan 23 isattached to and driven by the low pressure turbine 19 via a shaft 26 andan epicyclic gearbox 30.

In use, the core airflow A is accelerated and compressed by the lowpressure compressor 14 and directed into the high pressure compressor 15where further compression takes place. The compressed air exhausted fromthe high pressure compressor 15 is directed into the combustionequipment 16 where it is mixed with fuel and the mixture is combusted.The resultant hot combustion products then expand through, and therebydrive, the high pressure and low pressure turbines 17, 19 before beingexhausted through the nozzle 20 to provide some propulsive thrust. Thehigh pressure turbine 17 drives the high pressure compressor 15 by asuitable interconnecting shaft 27. The fan 23 generally provides themajority of the propulsive thrust. The epicyclic gearbox 30 is areduction gearbox.

An exemplary arrangement for a geared fan gas turbine engine 10 is shownin FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26,which is coupled to a sun wheel, or sun gear, 28 of the epicyclic geararrangement 30. Radially outwardly of the sun gear 28 and intermeshingtherewith is a plurality of planet gears 32 that are coupled together bya planet carrier 34. The planet carrier 34 constrains the planet gears32 to precess around the sun gear 28 in synchronicity whilst enablingeach planet gear 32 to rotate about its own axis. The planet carrier 34is coupled via linkages 36 to the fan 23 in order to drive its rotationabout the engine axis 9. Radially outwardly of the planet gears 32 andintermeshing therewith is an annulus or ring gear 38 that is coupled,via linkages 40, to a stationary supporting structure 24.

Note that the terms “low pressure turbine” and “low pressure compressor”as used herein may be taken to mean the lowest pressure turbine stagesand lowest pressure compressor stages (i.e. not including the fan 23)respectively and/or the turbine and compressor stages that are connectedtogether by the interconnecting shaft 26 with the lowest rotationalspeed in the engine (i.e. not including the gearbox output shaft thatdrives the fan 23). In some literature, the “low pressure turbine” and“low pressure compressor” referred to herein may alternatively be knownas the “intermediate pressure turbine” and “intermediate pressurecompressor”. Where such alternative nomenclature is used, the fan 23 maybe referred to as a first, or lowest pressure, compression stage.

The epicyclic gearbox 30 is shown by way of example in greater detail inFIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38comprise teeth about their periphery to intermesh with the other gears.However, for clarity only exemplary portions of the teeth areillustrated in FIG. 3. There are four planet gears 32 illustrated,although it will be apparent to the skilled reader that more or fewerplanet gears 32 may be provided within the scope of the claimedinvention. Practical applications of a planetary epicyclic gearbox 30generally comprise at least three planet gears 32.

The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3is of the planetary type, in that the planet carrier 34 is coupled to anoutput shaft via linkages 36, with the ring gear 38 fixed. However, anyother suitable type of epicyclic gearbox 30 may be used. By way offurther example, the epicyclic gearbox 30 may be a star arrangement, inwhich the planet carrier 34 is held fixed, with the ring (or annulus)gear 38 allowed to rotate. In such an arrangement the fan 23 is drivenby the ring gear 38. By way of further alternative example, the gearbox30 may be a differential gearbox in which the ring gear 38 and theplanet carrier 34 are both allowed to rotate.

It will be appreciated that the arrangement shown in FIGS. 2 and 3 is byway of example only, and various alternatives are within the scope ofthe present disclosure. Purely by way of example, any suitablearrangement may be used for locating the gearbox 30 in the engine 10and/or for connecting the gearbox 30 to the engine 10. By way of furtherexample, the connections (such as the linkages 36, 40 in the FIG. 2example) between the gearbox 30 and other parts of the engine 10 (suchas the input shaft 26, the output shaft and the fixed structure 24) mayhave any desired degree of stiffness or flexibility. By way of furtherexample, any suitable arrangement of the bearings between rotating andstationary parts of the engine (for example between the input and outputshafts from the gearbox and the fixed structures, such as the gearboxcasing) may be used, and the disclosure is not limited to the exemplaryarrangement of FIG. 2. For example, where the gearbox 30 has a stararrangement (described above), the skilled person would readilyunderstand that the arrangement of output and support linkages andbearing locations would typically be different to that shown by way ofexample in FIG. 2.

Accordingly, the present disclosure extends to a gas turbine enginehaving any arrangement of gearbox styles (for example star orplanetary), support structures, input and output shaft arrangement, andbearing locations.

Optionally, the gearbox may drive additional and/or alternativecomponents (e.g. the intermediate pressure compressor and/or a boostercompressor).

Other gas turbine engines to which the present disclosure may be appliedmay have alternative configurations. For example, such engines may havean alternative number of compressors and/or turbines and/or analternative number of interconnecting shafts. By way of further example,the gas turbine engine shown in FIG. 1 has a split flow nozzle 18, 20meaning that the flow through the bypass duct 22 has its own nozzle 18that is separate to and radially outside the core engine nozzle 20.However, this is not limiting, and any aspect of the present disclosuremay also apply to engines in which the flow through the bypass duct 22and the flow through the core 11 are mixed, or combined, before (orupstream of) a single nozzle, which may be referred to as a mixed flownozzle. One or both nozzles (whether mixed or split flow) may have afixed or variable area.

Whilst the described example relates to a turbofan engine, thedisclosure may apply, for example, to any type of gas turbine engine,such as an open rotor (in which the fan stage is not surrounded by anacelle) or turboprop engine, for example. In some arrangements, the gasturbine engine 10 may not comprise a gearbox 30.

The geometry of the gas turbine engine 10, and components thereof, isdefined by a conventional axis system, comprising an axial direction(which is aligned with the rotational axis 9), a radial direction (inthe bottom-to-top direction in FIG. 1), and a circumferential direction(perpendicular to the page in the FIG. 1 view). The axial, radial andcircumferential directions are mutually perpendicular.

The present disclosure will now proceed in relation to a gas turbineengine, however it will be appreciated that the present disclosure maybe used in other types of axial flow engine/machine.

As shown in FIGS. 4 and 5, the gas turbine engine 10 comprises aplurality of acoustic sensors 42. Preferably, the acoustic sensors 42comprise ultrasonic sensors. The ultrasonic sensors 42 are provided on acasing 44 surrounding and bounding the air intake 12 and the intake airflow 48 of the engine 10. The casing 44 may comprise an inner surface ofthe nacelle 21. The intake should be considered any area upstream of theengine compressor(s), e.g. upstream of the fan 23.

In an example, between two and forty sensors are provided. Typically,greater than four sensors would be used for suitable coverage over theflow area. The sensors may be spaced around the axis of rotation, e.g.as a circumferential array, and may or may not be equally spaced.However, it can be appreciated that increasing the number of sensors mayincrease the fidelity & accuracy of the measurements. The invention istherefore not limited to such an example, and any number of sensors maybe used as required, depending on the application.

The ultrasonic sensors 42 comprise an ultrasonic transmitter and/or anultrasonic receiver. Each individual sensor 42 may comprise atransmitter, receiver or a transmitter/receiver pair.

The ultrasonic transmitters comprise an ultrasonic transducer configuredto transmit an ultrasonic waveform 46 into the intake airflow 48.

The ultrasonic receivers are configured to receive and detect theultrasonic waveforms 46 transmitted by the ultrasonic transmitters. Inan example, the receivers are located in substantially the same locationas the transmitters and/or are formed integrally with the transmitters(i.e. they form part of the same assembly). In other examples, thereceivers are located in different locations to the transmitters and/orare formed as a separate assembly to the transmitters.

The sensors 42 are removed from the intake airflow 48 (i.e. they do notprotrude into, obstruct or otherwise interfere with the airflow 48). Thesensors 42 may be mounted on, or behind, the outer surface of the casing44 (i.e. within the nacelle 21 casing) and/or may be mounted flush withthe inner surface of the surface of the casing 44 (e.g. so that anedge/side of the sensor is flush with the gas-washed surface).

In other examples, the sensors 42 may protrude into the airflow. Thismay intentionally create turbulence, for example, to study airflowproperties within the engine.

The ultrasonic sensors 42 are spaced about the circumference of thecasing 44, preferably in an evenly distributed manner. The ultrasonicsensors 42 may be spaced about casing 44 such that each sensor 42 isdiametrically opposed another sensor 42. Alternatively, the sensors 42may be unevenly distributed, for example, in clusters. The sensors maybe angularly spaced about the axis 9.

The ultrasonic sensors 42 are located in and/or oriented in a singleplane (e.g. each of the individual sensors lies in an imaginary planebounded by the other sensors). The plane may be substantially flat. Inother examples, the plane arcuate, curved, or the like. The exact formof the plane is not pertinent to the disclosure at hand, however, itshould be appreciated that providing the sensors in a single plane meansthat the sensors are circumferentially spaced about the intake.Therefore, no two sensors within a given system are placed in the samecircumferential position (i.e. no two sensors are only axially spacedapart without circumferential spacing).

The plane in this example is substantially orthogonal to net directionof the local airflow 48. In other embodiments, the entire plane and/orportions of the plane are non-orthogonal to net direction of the localairflow 48.

In an example, the plane is substantially orthogonal to the principalengine axis 9. However, the plane could be offset, e.g. obliquely, fromorthogonal to the airflow/axis if desired.

In the example of a flat plane, one or more line of sight between theplurality of respective sensors 42 may be oriented to lie within asingle plane. All transmitters/receivers may be arranged totransmit/receive signals within the single plane.

The ultrasonic sensors 42 are located in an upstream portion of theengine 10, upstream of the compressor stage of the engine. The sensors42 are upstream of the fan 23. The sensors 42 may be located at,adjacent or closely behind the intake 12. In this example, the sensors42 are between an intake throat 50 (i.e. the narrowest point of theintake 12) and a fan casing 52 (i.e. the portion of the casing 44surrounding the fan 23).

In an example, the sensors 42 are offset in a downstream directionrelative to the intake throat 50. In other examples, the sensors 42 arelocated upstream of the intake throat. In any examples, upstream and/ordownstream directions may be assumed to be directions along axis 9 orparallel thereto.

The engine 10 may comprise an acoustic liner 54 surrounding the casing44 and configured to reduce acoustic vibrations therein. The acousticliner is located adjacent and/or upstream an upstream side of the fancasing 52. The sensors 42 may be located upstream of the acoustic liner54.

The sensors 42 may be located upstream or downstream of other sensingequipment located on the casing 44. The other sensing equipment maycomprise one or more of: a Pitot tube 56; or static pressure tube 58; ora temperature probe.

The engine may comprise an internal component located within the casing44, i.e. a solid region within the flow field. The internal componentmay axially extend through/within the casing 44. For example, theinternal component may comprise a portion of the rotor hub (forattachment of the fan blades 23 to the shaft); the spinner/nose cone 60;or a static portion/casing of the core 11 of the engine.

In an example, the sensors 42 are located upstream of the upstreamend/tip of the rotor hub and cone, such that the nose cone 60 does notintercept the plane of the sensors 42. As shown in FIG. 6, thepositioning of the sensors 42 prevents interruption of the line of sight62 between the plurality of sensors 42, such that each sensor 42 has aline of sight 62 to each of the other sensors 42, thereby allowingultrasonic communication between the sensors 42 in a substantiallystraight path.

Each line of sight 62 between the sensors 42 can be sampled to determinethe average properties of the airflow 48 along the lines of sight 62(i.e. the ultrasonic sensors measure an average value of the airflowproperties between the sensors and not only at a single point proximalthe sensors 42 themselves). The properties of the airflow 48 can besampled along a plurality of the lines of sight 62 between each of theplurality of sensors 42, to provide a plurality of samples across theairflow 48. The airflow properties are sampled at multiple spatiallyseparated points within the airflow, for example, including boundaryflow layers adjacent the surface of the casing 44.

As shown in FIG. 6, the plurality of samples provides a ‘mesh’ ofsamples across the airflow 48, which spans many regions of the flowfield/area.

In a different example, the sensors 42 are located downstream of thecone 60 tip, such that the cone 60 or rotor hub is present within (i.e.crosses) the plane of the sensors 42. As shown in FIG. 7, lines of sight62 between opposing sensors 42 are interrupted in an area bounded by thesolid region of spinner cone 60, thus preventing ultrasoniccommunication between those opposing sensors 42.

Due to the interruption of communication between the opposing sensors42, a deadzone 64 is created (e.g. around the solid body of the cone 60)where measurement of the air flow 48 properties cannot be performed. Itcan be appreciated that such a problem is present when the internalcomponent comprises other portions of the engine 10, for example, thecore 11 of the engine. Such a deadzone 64 may or may not be acceptablein different implementations. For example, if the number of sensors 42mounted about the casing is increased, the deadzone area may be reducedsufficiently.

FIG. 8 shows an alternative example arrangement for use when a solidinternal component or body is located in the plane of the sensors. Atleast one further acoustic sensor 66 is provided on the internalcomponent 60. The further sensor may comprise an ultrasonic sensor 66configured to operatively communicate with the ultrasonic sensors 42provided on the casing 44. In an example, between 1 and 6 furthersensors 66 are provided. However, it can be appreciated that in anynumber of further sensors 66 may be provided depending on theapplication.

The casing 44 surrounds the internal component 60. The further sensor 66is thus provided radially inward from the sensors 42 provided on thecasing 44.

The further sensors 66 may be evenly distributed about the circumferenceof the internal component. In a similar fashion to the sensors 42 on thecasing 44, the further sensors 66 may be removed from the air flow 48.

As shown in FIG. 8, line of sight 62 is maintained between the sensors42 on the casing 44 and the sensors 66 on the internal component,thereby eliminating and/or decreasing the size of the deadzone 64. Theline of sight between sensors 42 and 66 passes through a boundary layerflow at the surface of the internal component 60 and so the contributionof the boundary layer to the overall flow profile can be accommodated.

In an example, the further sensor 66 is affixed to the rotor/compressorfan hub (e.g. a cone 60 thereof), such that the further sensor 66rotates with the rotation of the hub, whilst maintaining operativecommunication with the sensor 42 provided on the casing 44. The furthersensor sensors thus rotate about the engine axis (albeit radially spacedtherefrom). In such an arrangement, the lines of sight 62 collectivelyspan substantially the entire flow area between the hub 60 and thecasing 44 in any given time frame.

As shown in FIG. 9, at time ‘t’, the further sensor 66 is in operativecommunication with a plurality of sensors 42 a, 42 b, 42 c on the casing44. The lines of sight 62 between the sensors are shown in the heavydashed lines. At time ‘t’, the properties of the airflow 48 along thelines of sight 62 can be sampled using the sensors 42 a, 42 b and 42 c.

As the further sensor 66 rotates with the hub, the further sensor movesto a new angular position at time ‘t+dt’ (dt being an arbitrarytimestep). The further sensor 66 communicates with the same plurality ofdifferent sensors 42 a, 42 b, 42 c provided on the casing 44, however,the lines of sight 63 (shown in light dashed lines) have moved to a newlocation. The properties of the airflow 48 along the lines of sight 63can be sampled using the same sensors, the portion of airflow 48 sampledat ‘t+dt’ being different from the portion of airflow 48 sampled at time‘t’.

As the further sensor 66 continues to rotate, the further sensor 66communicates with the next plurality of sensors 42 sequentially aboutthe casing (i.e. 42 b, 42 c, 42 d, then 42 c, 42 d, 42 e and so on). Asthe further sensor 66 rotates, the lines of sight 63 sweep through theairflow 48 within the casing 44. The properties of the airflow 48 aresampled throughout the rotation of the further sensor 66, thus providinga ‘sweeping scan’ of the air flow 48 surrounding the hub. The exampleshown in FIG. 9 allows a single further sensor 66 to sample the airflowregion.

In further examples, the cone 60 or hub may comprise a plurality offurther sensors 66, in a similar fashion to the sensors 42 on the casing44 or the further sensors 66 shown in FIG. 8. The plurality of furthersensors 66 may be distributed evenly about the circumference of the hub60. Such an arrangement of sensors allows all regions of the flow areato be scanned concurrently and in a sweeping manner during eachrevolution.

In some examples, the static lines of sight between static sensors 42may also be used for readings (e.g. as shown in FIGS. 7 and 8). Thetotal readings may thus comprise, in part, readings taken for a staticframe of reference and, in part, readings from a rotating frame ofreference.

As shown in FIG. 10, the sensors 42 are operatively connected to a powersource 74 and a signal processing system 72. The static sensors 42 mayhave direct electrical (e.g. wired) connection to the signal processingbox 72.

The engine 10 comprises a further sensor system 76, e.g. a telemetrysystem, operatively connected to a power source 78 and the signalprocessing system, 72. The further sensor system may be located upstreamof the sensors 42, e.g. in the nacelles and/or engine intake. Thefurther sensor system 76 is configured to receive the data measured bythe rotating sensors 66 wirelessly. The further sensor system thenforwards the data to the signal processing box 72.

A rotational electrical coupling 68 may be provided for the furthersensors 66. The rotational electrical coupling 68 connects the furthersensor 66 to a power source 70 and/or the signal processing system 72,and permits rotation of the further sensor 66 relative the power source70 and/or the signal processing system 72 whilst maintaining theconnection therebetween. The rotational electrical coupling 68 mayprovide a physical connection (e.g. a wire). In other examples, therotational electrical coupling 68 comprises wireless transmission (e.g.wireless power or signal transmission).

In some embodiments, the further sensors 66 may be directly operativelyconnected to the signal processing system 72, thus mitigating the needfor the further sensor system 76.

In an example, the engine 10 comprises a second plurality of sensors.The second set of sensors may be located a different axial location onthe engine 10 to the first plurality of sensors.

In some examples, the second plurality of sensors may be located in adownstream portion of the engine 10, preferably, at an exit nozzle ofthe engine 10. The second plurality of sensor may be axially spaced fromone another and may not be circumferentially spaced from one another.The second plurality of sensors may perform substantially the same wayas described in EP 3255438 A1, incorporation herein by reference.

The processing system 72 is configured to received signals from one ormore of: the first plurality of sensors 42; the further sensor(s) 66; orthe second plurality of sensors. The processing system 72 comprises oneor more computer processor configured to process the signals tocalculate the airflow velocity profile, the volumetric flow rate and/orthe mass flow of the intake flow 48.

The processing system 72 may be configured to provide signals to theultrasonic sensors to begin/end ultrasonic transmission and/orreception.

The processing system 72 is in operative communication with the furthersensor system 76. The further system 76 may provide values of one ormore operational parameter (i.e. values of one or more variableoperational parameter) required to calculate volumetric and/or the massflow rate of the intake flow 48.

The processing system 72 may be configured to log the airflow velocity,volumetric flow and/or the mass flow data over a given period of time.The processing system 72 may analyse the data to provide trends orpatterns therein (for example, using regression analysis) according tospecific parameters of the engine 10 or engine usage 10 (for example, aparticular power or thrust output of the engine 10 or a throttlesetting).

The processing system 72 may have an output interface configured to sendthe data relating to any of the processing inputs or outputs describedherein to a further system, such as a monitoring and/or control systemfor the engine or a subassembly thereof. The further system could beon-board the engine or aircraft, e.g. connected thereto by a data bus ora local wired or wireless network, or else a remote monitoring facility.The output of the processing system 72 could be used: for feedback to auser, e.g. a user interface in an aircraft cockpit; as an input for anoperational control system; and/or as an input for an equipment healthmonitoring system.

Additionally or alternatively, the processing system 72 comprisesnon-volatile memory for onboard storage of the data.

In some examples, additional conventional measurement devices may beprovided to determine the airflow properties in the engine. Theconventional devices may be used concurrently with the present system todetect/measure any difference between the two measurement techniques.

Calculation of the Mass Flow of the Intake Airflow

The following mathematical formulation estimates the flow velocityand/or the volumetric-flow of the intake flow 48 of an engine 10 with aknown stagnation temperature. Mass-flow rate can be estimated withadditional knowledge of stagnation pressure.

Nomenclature:

U: velocity of acoustic signal along line-of-sight between transmitterand receiver

V: flow velocity

m: mass-flow

M: flow Mach number

T: flow temperature

h: enthalpy

C: correction factor

α: velocity of sound

β: angle

s: distance

γ: adiabatic index

A=area

ρ=density

p=pressure

R: molar gas constant per molar mass of air

( )_(t): total or stagnation property, e.g. pressure and/or temperature

( )_(s): static property, e.g. pressure and/or temperature

( )_(TOF): Time-of-flight averaged quantity

( )_(m): mass-averaged quantity

( )_(eng): overall engine parameter

( )_(cr): core engine parameter

( )_(aux): auxiliary

( )_(thm): thermodynamic averaging

FIG. 11 shows a schematic of a plurality of sensors 42 configured tomeasure the velocity flow rate of the intake flow 48 within a casing 44.

An ultrasonic transmitter 80 transmits an ultrasonic waveform 46 intothe airflow 48. The ultrasonic waveform 46 interacts with the airflow 48and the speed the waveform travels through the airflow 48 variesaccording to various physical characteristics of the airflow 48, as willbe described below.

An ultrasonic receiver 82 is located within line of sight 62 of thetransmitter. The ultrasonic waveform 46 is received by the ultrasonicreceiver 82 and the time between transmitting the ultrasonic waveform 48and the receiving the waveform is calculated by the processing system 72to provide a measured time-of-flight (t_(TOF)).

Given a distance D of the line of sight between the ultrasonictransmitter 80 and receiver 82 and the measured time-of-flight (t_(TOF))of the acoustic signal, the time-of-flight averaged flow velocity(V_(TOF)) can be calculated as:

$\begin{matrix}{t_{TOF} = {\left. {\int\frac{ds}{\left( {{\alpha \cdot \overset{\rightarrow}{n}} + {\overset{\rightarrow}{V}}_{TOF}} \right) \cdot \overset{\rightarrow}{k}}}\Rightarrow V_{TOF} \right. = {\left. {f\left( {\beta,D,t_{TOF},a} \right)}\Rightarrow V_{TOF} \right. = {f\left( {\beta,D,t_{TOF},T_{s}} \right)}}}} & {{Eq}(1)}\end{matrix}$

-   -   given that a=√{square root over (gRT_(s))} modeling air as a        perfect gas.

With reference to FIG. 12, in a first step 200, equation (1) is used todetermine the mean time-of-flight-averaged velocity along the respectiveline-of-sight 62 between the transmitter 80 and receiver 82. This stepis repeated along each line-of-sight 62 between all of the respectivetransmitters 80 and receivers 82 in plane as required.

In a second step 202 and third step 204, once thetime-of-flight-averaged velocity of a desired selection/number oflines-of-sight has been calculated, tomography is then applied to derivethe time-of-flight-averaged velocity at one or more node; the node beingdefined by the intersection between two or more lines-of-sight 62.

The output from tomography is the spatial flow velocity profile on thesensor plane. For example, tomography provides a map of the flowvelocity profile across the sensor plane. Flow velocity at each node isof time-of-flight currency.

Given the velocity profile, a weighting correction can be applied to thenodes where velocity has been derived, to convert to the appropriatethermodynamic currency. In a fourth step 206, athermodynamically-weighted velocity (v_(thm)), e.g. mass-weighted, isdefined using a correction coefficient C₁ for velocity V_(TOF) at eachnode:

V _(thm) =C ₁ ·V _(TOF)   Eq(2)

In a fifth step 208, the calculation of static temperature T_(s) at eachnode is derived using knowledge of stagnation temperature T_(t) and theflow velocity V_(thm) from eq(2). Stagnation temperature T_(t) at theinlet is known based on flight conditions, or from aircraft or enginemeasurements. Stagnation temperature T_(t) has the advantage of having auniform profile across the inlet and hence the sensor plane, in absenceof exhaust gas re-ingestion.

$\begin{matrix}{{{h\left( T_{t} \right)} - {h\left( T_{s} \right)}} = {\left. \frac{V_{thm}^{2}}{2}\Rightarrow T_{s} \right. = {h^{- 1}\left( {{h\left( T_{t} \right)} - \frac{V_{thm}^{2}}{2}} \right)}}} & {{Eq}\mspace{14mu}(3)}\end{matrix}$

Steps 200 to 208 may be interactively repeated for each the nodes, untilconvergence to a tolerance.

In a sixth step 210, the calculation of local flow Mach number M at eachnode is known by application of its defining equation:

$\begin{matrix}{M = \frac{V_{thm}}{\sqrt{\gamma\;{RT}_{s}}}} & {{Eq}(4)}\end{matrix}$

In a seventh step 212, estimation of the intake mass flow rate isachieved by spatial integration of the non-dimensional mass-flowequation across the sensor plane in the intake:

$\begin{matrix}{{\overset{.}{m}}^{\prime} = {\int{\frac{p_{t} \cdot {f(M)}}{\sqrt{T_{t}}} \cdot {dA}}}} & {{Eq}(5)}\end{matrix}$

In an eight step 214, the estimated intake mass-flow rate is correctedfor sampling error using factor C₂, given that the nodes are samplingdiscrete points within the profile. The sampling correction may becalculated on the basis of a database during flight or post flight. Thecalculation of the sampling correction C₂ may use computational methodssuch as Computational Fluid Dynamics (CFD), or other methods aiming atresolving the flow regime to a required accuracy:

{dot over (m)}={dot over (m)}′·C ₂   Eq(6)

Calculation of the Bypass Airflow Mass

Referring to FIG. 13, the system may be used to determine the mass flowof the bypass air flow B through the engine 10. Bypass mass-flow can bederived from engine mass-flow (m_(eng)) by knowledge of mass-flow of thecore airflow A (m_(cr)). Core mass-flow in-flight can be estimated basedon methods calibrated at sea level and/or determined using otherconventional methods. The bypass airflow is thus the difference betweenthe total mass-flow into engine and the core mass-flow:

m ₁₂₅ =m _(eng) −m _(cr)   Eq(7)

Where m₁₂₅ is the bypass airflow at station 125. Station 125 is locateddownstream of the fan 23, preferably, between the fan 23 and an outletguide vane 84.

Station 150 is provided downstream of the OGV 84. The mass-flow of thebypass air B at station 150 is equal to the mass flow measured atstation 125 with the addition/subtraction of mass-flow due to sources orsinks (e.g. leaks, flow addition or subtraction from auxiliary systems,etc.). The mass flow of such sinks, sources and leaks can be modeled andconsidered known.

Therefore, the mass flow measured at station 150 be determined by:

m ₁₅₀ =m ₁₂₅ −m _(leak,1) −m _(aux)   Eq(8)

Calculation of the Bypass Stagnation Pressure at Charging Plane

The mass flow at station 150 can be used to determine the stagnationpressure p_(t) via equations (9) and (10):

$\begin{matrix}{\frac{m\sqrt{T_{t}}}{p_{t} \cdot A} = {\sqrt{\frac{\gamma}{R}} \cdot {f\left( \frac{p_{t}}{p_{s}} \right)}}} & {{Eq}(9)}\end{matrix}$

which can then be solved to determine the stagnation pressure p_(t) as afunction of the other parameters:

p _(t) =f(m, p _(s) , T _(t) , A, γ, R)   Eq(10)

-   -   The mass-flow m is derived by anemometry at the intake as per        the previously described method.    -   The static pressure p_(s) is measured at station 150    -   Stagnation temperature T_(t) downstream of the fan 23 may be        derived from engine analysis of shaft power, assumed based on        the fan characteristics or measured using conventional        techniques.    -   γ, R are known gas properties of the gas, typically air.    -   A is the geometric (cross-sectional) area at station 150, which        can be measured or known from design parameters etc. Corrections        to in-flight conditions may be applied to account for expansion        and/or contraction due to thermal or mechanical stresses etc.

Given that the geometric area at station 150 is considered in eq(10),the derived stagnation pressure is the average stagnation pressureacross the passage and all associated flow features, i.e. includingboundary layers and secondary flows, if any.

This completes the estimation of the stagnation pressure at chargingplane. The calculation of gross thrust can be completed using variouspublished gas path methods.

Calculation of In-Flight Nozzle Discharge Coefficient

Alternatively a new gas method is shown below, which focuses on thederivation of the nozzle discharge coefficient during engine operation.

Station 180 is provided at an outlet of the bypass airflow, for example,at the bypass nozzle throat. Therefore, the ratio of the mass-flow atstation 150 m₁₅₀ and station 180 m₁₈₀ can be determined by:

$\begin{matrix}{{\frac{m_{150}}{m_{180}} \cdot \sqrt{\frac{T_{t,150}}{T_{t,180}}} \cdot \frac{p_{t,180}}{p_{t,150}} \cdot \frac{A_{g,180}}{A_{g,150}} \cdot C_{d,180}} = {w\frac{f\left( {p_{t,150}\text{/}p_{s,150}} \right)}{f\left( {p_{t,180}\text{/}p_{s,180}} \right)}}} & {{Eq}(11)}\end{matrix}$

Where

-   -   C_(d,180) is the nozzle discharge coefficient.    -   Static pressure at station 150 p_(s,150) is measured by        conventional means.    -   Static pressure at station 180 p_(s,180) is typically called        “nozzle base pressure”. The nozzle base pressure may be        considered equal to ambient static pressure. Alternatively, a        correction on the ambient static pressure may be applied.    -   The stagnation pressures at station 150 p_(t,150) and station        180 p_(t,180) are considered to be equal by convention in the        existing gas path methods.    -   Geometric areas at stations 150 A_(g,150) and station 180        A_(g,180) are measured on ground or from known design        parameters. Corrections to in-flight conditions may be applied        to account for expansion and/or contraction due to thermal or        mechanical stresses etc.    -   Stagnation temperature at station 150 T_(t,150) and station 180        T_(t,180) is conserved in the absence of heat transfer.        Alternatively, any heat sources or sinks between stations 150        and 180 can be accounted for as is conventional.

The mass flow ratio,

$\frac{m_{150}}{m_{180}},$

between stations 150 and 180 is dictated by any mass sources and/orsinks between the stations. In a typical civil turbofan application, aleakage may exist, for example, through the thrust reverser and/ornacelle seals. The amount of leakage can be identified by pod leakagetests carried out on ground.

Thus, the nozzle discharge coefficient C_(d,180) can be determinedduring testing or flight etc. The determined value may then be comparedwith calculated or modeled values. The difference between measured andexpected value thus may indicated effects due to the nozzle, forexample:

-   -   External aerodynamic effects, otherwise called installation        effects, between the wing and the engine, such as nozzle        suppression effects.    -   Internal aerodynamic effects. Such effects may be profile        differences as observed within the engine environment to the        profiles tested on a rig, or different levels of turbulence        intensity, etc.

Calculation of In-Flight Thrust

The bypass thrust FG may then be calculated as per published gas pathmethods, using the charging plane thermodynamic parameters anddownstream nozzle performance coefficients, with mass-flow beingindependently known as per equation (8), stagnation pressure beingderived from eq(10) and the in-flight nozzle discharge coefficientderived from equation (11).

Given the one-to-one correlation of velocity to non-dimensionalmass-flow to nozzle pressure ratio at any given flight condition, anyrepresentation of flow through parameters involved in equations (1) to(6) can be used as a power setting parameter representing thrust.

Advantages of the Invention

The present disclosure provides a means to measure the airflowproperties of an axial flow engine with minimal intrusion into theairflow.

The present disclosure allows a greater number of sensors to be used, inorder to increase the accuracy of the measurement of the properties ofthe airflow.

The present disclosure provides an airflow measurement system with areduced sensitivity to the aerodynamic qualities of the airflow (i.e.the variability in the radial and circumferential profile, the amount ofturbulence etc.).

The present disclosure provides a measurement system more representativeof the average properties of the air flow through the engine.

The present disclosure provides a cross-sectional profile of the airflow through the system. This permits tomographic imaging of theair-flow profile using a series of measurements.

Only a single row/plane of sensors simplifies installation demands andrequires installation at the intake of the machine for accurateknowledge of flow thermodynamic properties, i.e. stagnation pressure,temperature, etc.

Mass flow rate can be determined in a practical and effective way usinga single row of acoustic sensors.

Location of the acoustic sensors adjacent/upstream of an acoustic linerfor the fan may be advantageous in filtering part of the pressure wavesemanating from the rotating fan tip.

Location of the flow sensors after the intake throat may take advantageof a more uniform flow profile.

Using a known stagnation temperature and pressure upstream of thecompressor (i.e. proximal the intake) allows accurate determination ofmass-flow and other airflow properties through the intake.

The inlet structure described herein may be part of a poddedinstallation, or an installation embedded within the airframe structure.

The present disclosure provides a non-intrusive means of measuring airflow.

Whilst the system and method is described in relation to a gas turbineengine, it could be applied to a wall/intake of any other suitableturbomachine, such as an axial flow machine, typically involving highsub-sonic flow rates and a strict requirement for aerodynamicefficiency.

It will be understood that the invention is not limited to theembodiments above-described and various modifications and improvementscan be made without departing from the concepts described herein. Exceptwhere mutually exclusive, any of the features may be employed separatelyor in combination with any other features and the disclosure extends toand includes all combinations and sub-combinations of one or morefeatures described herein.

We claim:
 1. A flow machine having a flow passage and an air flowmeasurement system, the air flow measurement system comprising: aplurality of acoustic sensors; the acoustic sensors comprising at leastone acoustic transmitter configured to transmit an acoustic waveformthrough the airflow passing through the flow passage to an acousticreceiver; and where at least one of the acoustic sensors is rotatablerelative to one or more other acoustic sensor.
 2. The flow machine ofclaim 1, where the flow machine comprises a compressor rotatable aboutan axis, the least one rotatable acoustic sensor being rotatable aboutsaid axis.
 3. The flow machine of claim 2, where at least one acousticsensor is provided at a first radial distance from said axis and atleast one acoustic sensor is provided at a second, greater radialdistance from said axis.
 4. The flow machine of claim 1, where at leastone acoustic sensor is mounted on a rotatable portion of the flowmachine within the flow passage and at least one acoustic sensor ismounted at a position surrounding said rotatable portion.
 5. The flowmachine of claim 4, where at least one acoustic sensor is mounted on acasing of the flow passage, and at least one acoustic sensor is mountedto a rotatable hub or shaft provided within the casing.
 6. The flowmachine of claim 5, where the rotatable hub comprises a compressor fanhub.
 7. The flow machine of claim 4, where a plurality of acousticsensors are mounted on the rotatable portion and/or a plurality ofacoustic sensors are mounted on the casing.
 8. The flow machine of claim7, where the plurality of acoustic sensors are circumferentially spacedon the rotatable portion and/or casing respectively.
 9. The flow machineof claim 7, where the respective lines of sight between each of theacoustic sensors provided on the rotatable hub and the acoustic sensorsprovided on the casing substantially span the entire flow area betweenthe hub and the casing at a given time frame.
 10. The flow machine ofclaim 4, where at least one acoustic sensor is recessed or mounted flushon the rotatable portion and/or casing.
 11. The flow machine of claim 1,where the acoustic sensors are provided at an intake of the flow passageand/or upstream of a rotor of the flow machine.
 12. The flow machine ofclaim 11, where the rotatable acoustic sensor is configured to sweep anarea of the intake.
 13. The flow machine of claim 12, where the at leastone rotatable acoustic sensor is configured to sweep an entire flow areabetween the hub and the casing during a revolution thereof.
 14. The flowmachine of claim 1, where the rotatable acoustic sensor is rotatableabout an axis substantially parallel to any or any combination of acentral axis of the flow passage; the direction of air flow through theflow passage and/or a rotational axis of the flow machine.
 15. The flowmachine of claim 1, where the acoustic sensors are all arranged in asingle plane.
 16. The flow machine of claim 1, where the at least oneacoustic sensor is wirelessly electrically coupled to allow electricalpower or electrical communication to be provided thereto.
 17. The flowmachine of claim 1, where the at least one acoustic sensor iselectrically coupled via rotating electrical interface to allowelectrical power or electrical communication to be provided thereto. 18.The flow machine of claim 1, wherein the flow machine is a turbomachineor a gas turbine engine.
 19. A system configured to determine air flowthrough a flow machine having a flow passage, the system comprising: aplurality of acoustic sensors, the acoustic sensors comprising at leastone acoustic transmitter configured to transmit an acoustic waveformthrough the airflow passing through the flow passage to an acousticreceiver, where at least one of the acoustic sensors is rotatablerelative to one or more other acoustic sensor; and a processing systemconfigured to receive signals from the acoustic sensors and determine aflow rate of the flow through the flow passage.
 20. A method ofdetermining air flow properties of an airflow through a flow passage ofa flow machine, the method comprising the steps of: providing signalcommunication with a plurality of acoustic sensors mounted relative tothe flow passage, where at least one of the acoustic sensors isrotatable relative to one or more other acoustic sensor; determining atime of flight of an acoustic waveform between said plurality ofacoustic sensors; and using the time of flight between the plurality ofsensors to determine an average flow velocity of the airflow through theflow passage.